In recent years, re-generation brake systems are used in vehicles such as a hybrid vehicle, an electric vehicle or the like that are equipped with a secondary battery. The re-generation brake systems regenerate electricity by using a motor which is usually employed for driving the vehicle. That is, in the re-generation brake system, the kinetic energy of the vehicle movement is collected as the generated electricity, and the electricity generation load is used for braking, i.e., for reducing the vehicle speed. The re-generation brake system for use in a vehicle charges a battery in the vehicle by the generated electricity, for the purpose of saving the energy. Therefore, a vehicle equipped with the re-generation brake system has an improved fuel mileage, in comparison to a vehicle with the mechanical brake only, by collecting the kinetic energy and reusing the collected energy.
However, the vehicular re-generation brake system has an upper limit of the regeneratable power (designated as the maximum re-generated power in the following description) due to the capacity of various parts such as the battery, charging device and the like in the vehicle. Therefore, if the generated electricity (i.e., the electric power, or the generated energy) by the motor exceeds the upper limit, the exceeding portion of the generated energy is converted to heat to be dissipated. That is, the exceeding portion of the generated energy is lost and cannot be collected as the electric energy.
In addition, the motor used in the re-generation braking has a peak generated electricity at a certain rotation speed, i.e., a maximum brake force exerted at that peak. Thus, the braking force from the re-generation braking has a certain upper limit. Therefore, the re-generation brake system in a vehicle is usually combined with a mechanical brake system that uses friction for braking.
As a result, the braking operation by the driver demanding a hard braking leads to the simultaneous operation of the re-generation braking and the mechanical braking. In that case, the mechanical braking dissipates the kinetic energy from speed reduction as heat from friction, thereby making it impossible to collect it as the electric energy.
In other words, the energy loss in the braking leads to the decrease in energy efficiency due to the energy loss that cannot be collected as electricity when the generated electricity of the motor exceeds the maximum re-generation power, or when the brake operation demands the braking force that exceeds the upper limit of the motor generated electricity.
On the other hand, as disclosed in a Japanese patent document 1, JP-A-2007-221889, a technique for improving energy efficiency of re-generation braking is known. In the vehicle driving support apparatus of the patent document 1, map information on speed reduction required points is extracted from a map database, and a target speed for a speed reduction point is set based on the extracted map information, and a required distance for reducing the current speed to the target speed by using the re-generation brake only is calculated. Then, a start timing of the brake operation is advised when the vehicle approaches a required distance from the speed reduction required point. In this manner, the driver can perform an efficient brake operation that causes only the re-generation braking for reducing the vehicle speed.
However, the technique in the patent document 1 has the following problem.
That is, guidance for re-generation braking can only be provided for the pre-defined points that are recorded as the map information, such as a stop sign, an intersection or the like. That is, guidance for the other points will not be provided, thereby not enabling the driver to perform the efficient brake operation at the other points. Further, the required distance for the braking prior to the stop sign or the intersection may be different driver to driver, depending on the driving habit. Therefore, the driving habits of the respective drivers must be considered for the guidance of re-generation braking, for further improving the energy efficiency.
Furthermore, the above technique can only provide the braking guidance in a feed-forward manner, which provides the guidance prior to the braking, without the evaluation of the actual braking performed under the provided guidance. Thus, training effects for the driver to perform an efficient brake operation based on the feedback of the actual braking will not be expected, and energy efficiency improvement effects on the re-generation braking can not be expected.
Furthermore, braking guidance is provided at the pre-defined points based only on the map information without taking the driving habit of the driver into consideration. Therefore, the driver will have redundant and/or annoying braking guidance for a specific point where he/she always minds for not losing an efficient re-generation braking timing, if the specific point is one of the pre-defined points.